Massive cosmic car crash revealed

Astronomers are studying what they decribe as one of the most complex and dramatic collisions between galaxy clusters ever seen, and which has taken 350 million years to play out.

The team gave Pandora's Cluster - officially Abell 2744 - its nickname because the collision has released so may strange phenomena, many of which have never been seen before.

"Like a crash investigator piecing together the cause of an accident, we can use observations of these cosmic pile-ups to reconstruct events that happened over a period of hundreds of millions of years," says
Julian Merten, one of the lead scientists.

"This can reveal how structures form in the Universe, and how different types of matter interact with each other when they are smashed together."

The cluster has now been studied in more detail than ever before by combining data from the NASA/ESA Hubble Space Telescope, ESO's Very Large Telescope (VLT), the Japanese Subaru telescope and NASA's Chandra X-Ray Observatory.

The team mapped the positions of all three types of matter in Abell 2744. Although the galaxies are bright, they make up less than five percent of the mass. Around 20 percent is hot gas, visible only in X-rays, and the rest is invisible dark matter.

The NASA/ESA scientists retraced the details of the collision, and deduced that at least four different galaxy clusters coming from a variety of directions were involved.

It seems that the complex collision has separated out some of the hot gas and dark matter so that they now lie apart from each other, and from the visible galaxies.

Near the core of the cluster is a 'bullet', where the gas of one cluster collided with that of another to create a shock wave. The dark matter passed through the collision unaffected.

In another area, there seem to be galaxies and dark matter, but no hot gas, as this may have been stripped away during the collision.

Even odder features lie in the outer parts of the cluster, says the team. One region contains lots of dark matter, but no luminous galaxies or hot gas. A separate ghostly clump of gas has been ejected, which precedes rather than follows the associated dark matter.

This is puzzling, say the scientists, but may tell us something about how dark matter behaves and how the various complonents of the universe interact with one another.